The present invention relates in general to new and improved pneumatic piston apparatus and in particular to an air-powered nail driver which employs a poppet valve that avoids the problems inherent in apparatus of this type and which interfaces with the piston in a novel and improved manner.
The use of poppet valves in pneumatic piston apparatus is well known, as evidenced by U.S. Pat. No. 3,952,398. Such valves generally have a movable member adapted for linear travel between pre-set limits in order to admit or to block the admission of a fluid, e.g., pressurized air. In present day applications, it is common for such valves to operate in an environment where the air pressure employed may exceed 200 psi. During the operation of the pneumatic piston apparatus the pressure may vary dynamically, sometimes in a different manner for different parts of the apparatus. Where the poppet valve includes composite members, i.e., members that include layers of different materials, delamination resulting in the separation of the layers of material may occur during the operation. Such action is particularly pronounced where the interface between such layers is exposed to the dynamically varying pressure conditions.
By way of example, in the above-mentioned U.S. Pat. No. 3,952,398 the movable poppet valve member terminates in a face plate which consists of a metal disk overlaid by a sealing pad. The purpose of the pad is to seal air out of the piston cylinder when the poppet valve member is in its downward position. Because of the dynamically varying pressure conditions that occur in the pneumatic piston driver illustrated in the patent, the air pressure acting on the exposed portion of the face plate during the operation of the apparatus has a tendency to force the resilient sealing pad away from its metal backing plate.
Such delaminating action has been observed in the operation of equipment of this type, notwithstanding the fact that the pad may be molded or cemented to its backing metal plate, or that it may be secured thereto by means of screws. Further, because the air pressure may act in a non-uniform manner on the exposed part, the delamination effect may occur at one point only, or it may occur at a number of points along the sealing pad. In either case, the poppet valve member is prevented from firmly seating on the cylinder in a manner where air is effectively sealed out. As a consequence, the piston driver may fail to operate efficiently, or it may be rendered completely inoperative by the delaminating action.
In apparatus of the type described, the applied air pressure normally urges the poppet valve member to one extreme of its axial travel to establish its rest position. In that position, the sealing pad forms a seal with the upper edge of the piston cylinder to prevent the application of the ambient air pressure to the upper surface of the piston. Simultaneously, air admitted to the underside of the piston urges the latter against the exterior surface of the sealing pad. When the piston driver is subsequently actuated and the poppet valve member moves to the opposite extreme of its travel to permit the application of pressurized air to the upper piston surface, the natural seal formed upon close contact between the exterior sealing pad surface and the upper piston surface may cause the piston to adhere to the face plate. When such adhesion occurs, the piston is sucked out of the piston cylinder with the upwardly moving poppet valve member and is incapable of executing its driving stroke. Thus, the operation of the piston apparatus becomes unpredictable.